Thesis Examination Committee
Prof Ning WANG, PHYS/HKUST (Chairperson)
Prof Kei May LAU, ECE/HKUST (Thesis Supervisor)
Prof Man Sun CHAN, ECE/HKUST
With a sizable bandgap and a high degree of electrostatic gate control, ultrathin two-dimensional (2D) molybdenum disulfide (MoS2) have recently gained tremendous interest for flexible electronic and optoelectronic applications. However, MoS2 based electronic devices suffer from several issues including lack of scalable methods to synthesize high-quality and large-area thin films, low electron mobility in MoS2 transistors, and difficulty in integration with high-k dielectrics. This thesis is devoted to developing MoS2/high-k top-gate transistors with high electron mobility through MoS2/dielectric stack optimization.
First, we investigate epitaxial growth of MoS2 on lattice-matched GaN. High-quality, unstrained, and few-layer MoS2 with strict registry to the GaN lattice is achieved. By combining atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and first-principles simulation, we qualitatively determine the interlayer coupling and atomic registry in MoS2/GaN stack. Second, we propose a HF passivation technique to improve the carrier mobility and interface quality of chemical vapor deposited (CVD) monolayer MoS2 on the SiO2/Si substrate. This passivation method leads to a more than doubled electron mobility, a reduced gate hysteresis gap, and a low interface trapped charge density, due to satisfied interface dangling bonds and reduced interface trap trapped charges. Finally, we investigate the integration of high-k dielectrics with 2D MoS2 by atomic layer deposition (ALD). Using a low temperature ALD method, we obtain uniform and conformal ZrO2 films on MoS2, with the corresponding top-gate MOSFET exhibiting good device performances comparable to that of HfO2/MoS2 transistor. We further study the feasibility of direct ALD growth of high-k oxides on CVD MoS2. Our investigation reveals that due to the overdeposition of molybdenum trioxide (MoO3) on the surface of MoS2, sub-10 nm high-k oxides can be easily deposited on CVD MoS2. These results may provide important scientific insights for achieving high performance MoS2 based nano-electronic devices.